The Research And Application Of Dual-axis Magnetoresistive Sensor Based On HMC1022

Magnetoresistive sensor using anisotropic magnetoresistance effect of ferromagnetic material permalloy (Fe20Ni80) is a kind of sensor which can measure the direction and magnitude of the magnetic field. The sensor has such advantages as small size, low power consumption, high sensitivity, high anti-interference ability, high reliability, easy installation etc. They not only show great advantages in the measurement of weak magnetic field and other areas based on weak magnetic field such as earth-magnetism navigation, digital smart compass, position measurement, identification of counterfeit banknotes, but also can be used to produce speed sensor with high-precision, pressure sensor, angular displacement sensors, and have broad application prospects.This paper conducts a detailed study on the operation principle and application of the magnetoresistive sensor. Analysis focused on the characteristics of HMC1022 and dual-axis magnetic field measurement circuit based on HMC1022 is emphasized, and the overall application circuit from magnetic induction to data show circuit is designed .In accordance with the design process, the entire measurement system is divided into five modules including t magnetic induction circuit, amplifying circuit, set/reset circuit, the circuit to eliminate bridge offset, data acquisition and display circuit. Certain modules are verified after theoretical research, calculation, simulation, welding circuit and experimental test. Therefore, main tasks of this paper are summarized as follows:1. The working principle and characteristics of AMR sensors are studied. On the Basis, the magnetic probe--magnetoresistive sensor HMC1022 from Honeywell Company in the design is learned more. In addition to having Wheatstone bridge structure, HMC1022 has not only a which can reduce the effect of temperature drift, non-linear errors and the loss of output signals caused by the existence of high magnetic field, but also a patented integrated offset current strip to eliminate the effects of hard iron interference. With the existence of these current strips, the ne ed for installation of the external coil is eliminated, increasing density of integration, reducing the uncertainties and improving the reliability and anti-interference ability. Due to HMC1022 with a unique internal structure, drift compensation circuit is not used in the circuit design, which makes the entire measuring circuit simple greatly.2. Magnetoresistive sensors convert magnetic field signal to electrical signal output by the Wheatstone bridge structure. Differential output voltage between two crunodes of wheatstone bridge represents the magnitude and direction of the magnetic field. According to the characteristic, chips of differential amplifiers and instrumentation amplifiers with the ability of differential enlargement are taken into account. Differential amplifier chip AMP04 which can be supplied by single power supply is selected to constitute amplifying circuit in this paper. Only 1k external resistor can easily adjust the amplification factor to 100; As a result of the frequency of the earth magnetic field during experiment testing close to DC, only a 15nF capacitor parallel internal 100k resistor form low-pass filter with the cut-off frequency of 100Hz, without external resistance and inductance in the amplification circuit. When the two differential input nodes are short-circuited and grounded, there is still 11mV offset voltage in the output side during the simulation with Or CAD PSpice9.2. There are two ways to deal with this offset voltage: the first one is adding regulation offset circuit to the input side; the second one is making the two readings subtract in the follow-up data processing. The three-terminal voltage regulator 78L05 provides power supply for amplifying chip in this article. However, there is certain equivalent serial resistance between the input side and the output side, which seriously affects the use of the regulation offset circuit. So the first approach is taken.3. The impacting current pulse is generated by use of the charge-discharge principle of switched-capacitor, completing operation of set and reset for sensors in order to facilitate directional magnetization into the thin-film magnetic components. Set / Reset pulse refers to transient current with large amplitude, and short duration generated with the clock controlling the charging and discharging of capacitance, mostly using switches or MOSFET or transistor which can act as switches. The power MOSFET IRF7105 integrated N-channel and P-channel act as a switch, N-channel and P-channel FET turn-on and cut-off by turns making capacitance charge and discharge turn to turn under the control of the clock in this paper. Because the magnitude and duration of pulse current impact set/reset operation, not only DC power supply of switch deciding to current magnitude, but also turn-on resistance and peripheral components impacting pulse duration should be considered. To achieve current pulse compliant with the circuit requirements, the appropriate components are chosen after theoretical analysis and calculation.4. Data processing and display circuit is one of the most important part in the entire measurement system. As a result of the specificity of the measurement method, data processing in the single chip is essential to the accuracy of measurement data. The two readings are mirror symmetrical after the sensor is set or reset. So two readings of every channel are respectively recorded after the set and reset pulse, making the two readings subtract in the single-chip. Real measurement data can be gotten after other relational mathematical operation in the data processing. This measurement method simplifies the circuit without considered fixed offset voltage.5. Processing the power supply of circuit and the input signal into ADC. As the output signal of the sensor is very small in the valid measuring range, in order to avoid noise submerging useful signal, filter out noise caused by DC power supply is necessary. Filtering method aiming at DC power supply is the use of large capacitors (electrolytic capacitor) parallel small capacitor (ceramic capacitors) in this paper. Because TLC2543 has a lower input impedance, buffers based on OP213 are used in the analog inputs, not only converting resistance but also preventing the noise signal from entering the analog inputs in this article.